This invention relates to a vehicle antiskid control system which prevents the vehicle body from making irregular turns with wheels locked when the brake is applied and is particularly concerned with a system wherein the brake pressure is reduced when the beginning of a locked state of wheels is detected; a minimum value of the wheel velocity is memorized; the memorized value is compared with the wheel velocity and the time for stopping the brake pressure reducing action is determined accordingly.
In the conventional antiskid systems of this kind, a wheel acceleration signal is obtained by differentiating the wheel velocity signal which is obtained from a pick-up attached to the periphery of the wheel axle. When the acceleration signal becomes less than a certain negative value, a locked state of wheels is considered to have begun and the brake pressure is reduced by actuating a pressure modulator. The wheels are considered to have been relieved from the locked state when the acceleration signal becomes greater than a certain positive value. Then the operation of the pressure modulator is stopped and the brake pressure is made to increase gradually. This cycle of control is repeated to prevent a lock of wheels until the brake action is completed.
However, a problem with such systems lies in noises that are caused by shocks resulting from uneven road surfaces and at the time of gear shifting. Such noises tend to cause erroneous control actions. Such noises, being in a pulse-like state, are greatly exaggerated by differentiation.
In order to avoid such erroneous actions, there have been contrived various methods for detecting the beginning of a locked state of wheels. In such methods, for example, the control cycle is not commenced by reducing the brake pressure when the wheel acceleration signal merely becomes less than a certain negative value, but the wheel velocity at that time is first memorized and then, whether or not it is an actual decrease in wheel velocity, is confirmed by checking the wheel velocity after a certain period of time before the commencement of the control cycle. However, also in connection with the prevention of the above stated erroneous actions, there has been known no suitable method for detecting the recovery of wheels from a locked state.